1. Field of the Invention
The present invention relates to a process for producing a cable, in particular for medium-tension or high-tension power transmission or distribution.
More particularly, the present invention relates to a process for producing a cable, preferably for medium-tension or high-tension power transmission or distribution, which has at least one coating layer made of a thermoplastic polymer material.
2. Description of the Related Art
The need for products that are highly environmentally friendly, obtained from materials that do not damage the environment, either during production or during use, and that are readily recyclable at the end of their life, is particularly felt even in the sector of power cables, telecommunications cables, data transmission cables and/or combined power/telecommunications cables.
However, the use of environmentally friendly materials is decidedly conditioned by the need to limit the costs and yet maintain performances, under the most common working conditions, that are equivalent to or even better than those of conventional materials.
In the sector of medium-tension or high-tension power transmission cables, the different coatings, which surround the conductor commonly consist of a crosslinked polymer material based on polyolefins, in particular crosslinked polyethylene (XLPE) or ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) elastomeric copolymers, which are also crosslinked. The crosslinking, carried out after the step of extruding the polymer material around the conductor, gives said material satisfactory mechanical performances, even under hot conditions in continuous use and under conditions of current overload. The polymer material is usually crosslinked via a radical route, by adding organic peroxides, or via a silane route, by means of introducing onto the polyolefin chains hydrolysable silane groups which, in the presence of water and of a suitable condensation catalyst give rise to the crosslinking.
However, it is well known that crosslinked materials are not recyclable, as a result of which both the process wastes and the cable coating material at the end of its life can be disposed of only by incineration.
Electrical cables are also known which have an insulation consisting of a multilayer winding of a paper or paper/polypropylene laminate impregnated with very large amounts of a dielectric liquid (commonly known as bulk-impregnated cables or oil-filled cables). By completely filling the spaces present inside the multilayer winding, the dielectric liquid prevents the occurrence of partial discharges and thus perforation of the electrical insulation. Dielectric liquids that are commonly used include products such as, for example: mineral oils, polybutenes, alkylbenzenes and the like (see for example patents U.S. Pat. Nos. 4,543,207, 4,621,302, EP-A-0 987 718 and WO 98/32137).
However, it is known that bulk-impregnated cables have many drawbacks compared with cables with extruded insulation, as a result of which their use is currently restricted to specific fields of application, in particular to the production of high-tension and ultra-high-tension direct-current transmission lines, both for terrestrial installations and, above all, for underwater installations. Specifically, the production of bulk-impregnated cables is very complex and expensive, not only because of the high cost of the laminates, but also because of the difficulties inherent to the winding steps of the laminate and its subsequent impregnation with the dielectric liquid. In particular, the dielectric liquids used are required to have a low viscosity under cold conditions so as to allow high-speed and uniform impregnation, and at the same time they are required to have little tendency towards migration during the laying and functioning of the cable, so as to avoid losses of said liquid from the cable ends or due to breakage. In addition, bulk-impregnated cables are not recyclable and their use is limited to working temperatures lower than 90° C.
In the context of power cables with a non-crosslinked extruded coating, the use of thermoplastic materials of different kinds has been proposed in the prior art.
For example, patent application WO 96/23311 discloses a low-voltage, high-current cable in which the insulating coating, the inner sheath and the outer sheath are made of the same non-crosslinked polymer material, coloured black by addition of carbon black. The use of the same material in the different layers does not require the abovementioned components to be separated in a recycling process. For a maximum working temperature of 90° C., the possibility of using heterogeneous thermoplastic elastomers consisting of a polypropylene matrix in which is dispersed an elastomeric phase consisting of EPR or EPDM copolymers is indicated.
Patent application EP-0 893 801, in the name of the Applicant, discloses a cable comprising a conductor and one or more coating layers, in which at least one of said coating layers comprises, as non-crosslinked base polymer material, a blend comprising: (a) a crystalline propylene homopolymer or copolymer; and (b) an elastomeric ethylene copolymer with at least one α-olefin containing from 3 to 12 carbon atoms, and optionally with a diene; said copolymer (b) being characterized by a 200% tension set value (measured at 20° C. for 1 minute according to ASTM standard D 412) of less than 30%.
Patent application EP-A-0 893 802, in the name of the Applicant, discloses a cable comprising a conductor and one or more coating layers, in which at least one of said coating layers comprises, as non-crosslinked base polymer material, a blend comprising: (a) a crystalline propylene homopolymer or copolymer; and (b) a copolymer of ethylene with at least one α-olefin containing from 4 to 12 carbon atoms, and optionally with a diene; said copolymer (b) being characterized by a density of between 0.90 and 0.86 g/cm3, and by a composition distribution index, defined as the percentage by weight of the copolymer molecules having an α-olefin content which is not more than 50% of the total average molar content of α-olefin, of greater than 45%.
Patent application WO 00/41187, in the name of the Applicant, discloses a cable comprising a conductor and at least one coating layer based on a non-crosslinked polymer material comprising a heterogeneous copolymer having an elastomeric phase based on ethylene copolymerized with an α-olefin and a thermoplastic phase based on polypropylene. The elastomeric phase in said heterogeneous copolymer represents at least 45% by weight relative to the total weight of the heterogeneous copolymer, and the heterogeneous copolymer is substantially free of crystallinity derived from polyethylene blocks. The elastomeric phase preferably consists of an elastomeric copolymer of ethylene and propylene which comprises from 15% to 50% by weight of ethylene and from 50% to 85% by weight of propylene, relative to the weight of the elastomeric phase.
Patent application WO 99/13477 discloses an insulating material consisting of a thermoplastic polymer forming a continuous phase which incorporates a liquid or readily fusible dielectric, which forms a mobile interpenetrating phase in the solid polymer structure. The weight ratio between the thermoplastic polymer and the dielectric is between 95:5 and 25:75. The insulating material can be produced by hot-blending the two components in a batchwise or continuous manner (for example by means of an extruder). The resulting blend is then granulated and used as insulating material for the production of a high-tension electrical cable by means of extrusion around a conductor. The material can be used either in thermoplastic form or in crosslinked form. Among the thermoplastic polymers the following are indicated: polyolefins, polyacetates, cellulose polymers, polyesters, polyketones, polyacrylates, polyamides and polyamines. It is particularly suggested to use polymers of low crystallinity. The dielectric is preferably a synthetic or mineral oil, of low or high viscosity, in particular a polyisobutene oil, naphthenic oil, polyaromatic oil, α-olefinic oil or silicone oil.
The Applicant believes, however, that the technical problem of obtaining an electrical cable with a coating consisting of a thermoplastic polymer material which has mechanical and electrical properties that are comparable with those of cables with an insulating coating made of crosslinked material, remains to be solved. In particular, the Applicant set itself the problem of producing a cable with a non-crosslinked insulating coating which has good flexibility and high mechanical strength under both hot and cold conditions, and at the same time high dielectric rigidity, without using potentially pollutant products during the life cycle of the cable, that is to say from its production to its disposal.
In view of the abovementioned problem, the Applicant has found that although the addition of dielectric liquids to thermoplastic polymer materials significantly increases the electrical properties of the material (in particular the dielectric rigidity), it presents many problems from the point of view of industrial implementation.
Specifically, the Applicant believes that the dielectric liquid added to the thermoplastic polymer material needs to maintain the properties of the material (thermomechanical properties, ease of handling) without giving rise to phenomena of exudation of said dielectric liquid. In addition, the dielectric liquid should distribute itself uniformly within the material, so as to ensure high electrical performances throughout its thickness. For example, when the coating made of thermoplastic polymer material is the insulating coating, it is important that the dielectric liquid should distribute uniformly itself throughout the coating thickness and should be present, in particular, in the interface zones between the inner and outer semiconductive layers usually present in a medium-tension and/or high-tension electrical power transmission and/or distribution cable. In this way, the resulting cable can ensure substantially constant performances over time, and thus a high level of reliability, even at elevated working temperatures (of at least 80° C., preferably of at least 90° C.).
In particular, the Applicant has found that the action of mixing the dielectric liquid into the thermoplastic material, which can take place during an extrusion process, does not make it possible to obtain a coating with a substantially homogeneous distribution of the dielectric liquid throughout its thickness. Specifically, the dielectric liquid tends to become concentrated in the inner zones of said coating, to the detriment of the outermost zones, which are actually the zones that are most sensitive to partial discharges and, thus, to perforation.